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Sun pillar

Sun pillar

The other morning, I saw a column of light above the rising sun — what was that?

A rising, or setting, sun behind high clouds will sometimes project a vertical shaft of light above it. This is called a sun pillar. The sun pillar is caused by reflection of sunlight by tiny ice crystals in a cloud that is near the horizon. Each of the flat-plate ice crystals floating in the cloud acts like a small mirror, reflecting the sun’s light to your eyes in a line above the sun. If the crystals were not there, the sunbeams that they are reflecting would never be seen by you. Those beams would be traveling far above your head.

This pillar of light often has an orange tint. This color arises because the blue and violet colors are removed from the sunlight through interactions with the air. Gas molecules in the air (mostly oxygen and nitrogen) interact with blue colors, scattering the blue light in all directions. This is why the cloud-free sky looks blue during the day. At sunset and sunrise the sun is on the horizon and the sun’s rays pass through more atmosphere than during the day. More atmosphere means more molecules to scatter the blue colors. With the longer path, all the blue and violet light gets redirected out to space while much of the yellow, orange and red colors remain. Ice crystals scatter any color. So, with the blue colors scattered out to space, the oranges reach the cloud and are reflected by the large flat-plate ice crystals, redirecting the light towards our eyes.

Steven A. Ackerman and Jonathan Martin are professors in the Department of Atmospheric and Oceanic Sciences at UW-Madison, are guests on the Larry Meiller‘s WHA-AM radio show the last Monday of each month at 11:45 a.m.